1. Field of the Invention
The present invention is related to thermal printers to be utilized for label printers, etc.
2. Description of the Related Art
At present, various kinds of printers are actually used, and thermal printers which are able to perform image printing quietly and have simple constitutions are utilized as label printers, etc., for example.
In a thermal printer as mentioned in the above, a line head is fixed to be capable of freely approaching to or receding from a platen to be able to mount a recording medium such as a label sheet.
In such a thermal printer, it is desired to make the pressure of a line head toward a platen variable to keep good printing quality even if the thickness of a recording medium is varied. There is an apparatus disclosed in a Japanese Patent Laid-Open No. Hei 1-8055 by the present applicant as a thermal printer in which such a mechanism as described in the above is realized.
A thermal printer of such a constitution will be explained referring to an example based on FIG. 9. In a label printer 1, a thermal printer as mentioned in the above, a platen roller 3 is supported by its shaft to be freely rotatable between a pair of supporting frames 2, and a line head 4 having heating elements (not shown in a drawing) on the surface being capable of freely approaching to or receding from the platen 3, together with a bracket 5, are supported to be freely slidable between a pair of supporting frames 2. In the label printer 1, a polygon cam 8, a variable pressure mechanism arranged to be freely rotatable being connected to a control lever 6 through a rotary shaft 7 outside the supporting frame 2 is abutting the bracket 5 from the upper side, and the bracket 5 is connected to the line head 4 through an elastic body, a spring (not shown in a drawing). In the label printer 1, the supporting frame 2, the control lever 6, etc. mentioned in the above are housed in a main body housing (not shown in a drawing).
In the label printer 1 having a constitution as mentioned in the above, a spring under the bracket 5 being positioned and held with the polygon cam 8 is pressing the line head 4 toward the platen roller 3; thereby, image printing is performed on a recording medium (not shown in a drawing) such as a thermosensible paper conveyed onto the platen roller 3 by heating scanning of the line head 4.
In the label printer 1, the thickness of a label sheet and that of a tag sheet, for example, to be utilized as recording media are different from each other; thereby, it is necessary to adjust the pressing force between the line head 4 and the platen roller 3 corresponding to the thickness of a recording medium. In the label printer 1, the pressing force of a line head 4 acting toward the platen roller 3 is made variable with the action of a spring under the bracket 5 in displacing the position of the bracket 5 relative to the supporting frame 2 stepwise in rotating the polygon cam 8 by manually operating the control lever 6.
In the label printer 1, the pressing force of the line head 4 toward the platen roller 3 can be varied by rotating the polygon cam 8 positioned on the bracket 5 which presses a spring against the line head 4. Therefore, there is the polygon cam 8 placed on the bracket 5, and the receding dimension of the line head 4 from the platen roller 3 is limited. Because of this, there is a problem that when a recording medium is loaded, the loading work is made difficult by the small space.
It is difficult to securely position and hold the bracket 5 being energized with a spring by the polygon cam 8, and there is a fear that the polygon cam 8 may be rotated by vibration, etc. and the bracket 5 can be displaced.
Moreover, in the label printer as described in the above, the control lever 6 is fixed on the rotary shaft 7 and is installed so as to penetrate the supporting frames 2 for rotating the polygon cam 8, so that the length of the rotary shaft 7 in the direction of the axis has to be longer than that of the line head 4, which prevents the label printer 1 from being small in size and light in weight.